Picture of Tube Amp Rebuild (and Mod)
Striving for that old school sound, you purchase a 'vintage' guitar amp. But it doesn't sound quite right. Well, any amp more than 20 years old is gonna need work...

What's the deal with tube amps? Why all the fuss?

Yes, they do have a special sound, one that digital 'modeling' can't quite get right... It's actually the limitations of tubes that impart that special tone--that natural compression and smooth breakup. Sure, they keep improving solid-state amplifiers--but a look at the majority of mid-range to high-quality amps, the current models (mostly all tube, tube/ss hybrids, etc.) should convince you that it's very difficult to capture 'that sound' without tubes.

Does a 'rebuild' ruin the value of my vintage equipment?

No. Maybe. I don't know. Does changing the tubes mean the amp is not longer 'vintage?' Every old amp will need a rebuild at one point or another. If this negates it's 'vintage' status, then there is no such thing as a vintage amp! Functional amps, anyway--o.k. for nutso collectors who don't actually use 'em.... Who cares about them!

This little jem is a 1961 Kay 503A instrument (guitar, harp) amplifier. Output is in the 3-4 watt range. A nice studio, or "living room amp."

Here's a taste, and there's more video on the last step:
#1 -- Mod A (my first attempt--If you only have patience to listen to one clip, play #2):

(recognize the Trash-o-caster?)

#2 -- This is Mod B, IMHO, a much better sound:
(pickups switched from middle to middle/neck phased mix, about half way)

#3 -- Quick one added, with a Gibson Les Paul, just to show the amp has some bluesy distortion..
(pretty 'raggedy' playing on this one, but it's enough to catch the sound...)

All the videos are 'clean'--guitar and amp only, no FX.

Danger! Danger! No, really, DANGER!!!!!

A tube amp, even an unplugged tube amp, stores enough electrical energy in it's capacitors to kill you! YES, KILL YOU. Don't touch it unless you take precautions. See the section on DISCHARGE THOSE CAPS!!!!!

I'm not a tube repair technician, or an expert electrician. I'm just a hobbyist. Don't take my word, do your own research, and please be careful !
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ksmith1351 year ago
Thank u
ksmith1351 year ago
Hey gmoon. Do u have any info on an Ampeg vt60 combo. I'm trying to bring it back to life and I'm having a hard time. Thanks.
gmoon (author)  ksmith1351 year ago

No direct experience. I know there's a VT120 schematic on Schematic Heaven, which is probably identical to the VT60, other than having double the output tubes...

ksmith1351 year ago
Hey gmoon. Ur helping me a lot.
gmoon (author)  ksmith1351 year ago

Cool. Be safe with this stuff...

DANIELEPO6 years ago
hi! I'm thinking of modding my amp and found this very helpfull. only one question: what you mean with high-end? I thought lowering those caps would increase the bass... thanks for all
gmoon (author)  DANIELEPO6 years ago
Hey, good question. I assume by saying "lowering those caps," you mean make smaller values, right?

General rule is that the smaller the cap value, the more it blocks low frequencies.

(jeeze, I see that I have several duplicate component names-- two C3, two C2, etc. Bear with me, I hope this makes sense with the lousy schematic..)

How that works to filter frequencies depends on how it's oriented in the circuit:

-- caps inline with the signal, like the 0.01uF (upper most cap labeled C3), are "high pass" filters. They block low frequencies from passing forward. These are called "coupling caps", and also block any DC voltage offsets from messing with the next stage.

Changing from .05 to .01 blocks more lows from passing.

-- caps that shunt or bleed signal to ground, like the 0.001 that's attached to the tone POT (lower C2) are "low pass" filters. Those caps block low frequencies from passing to the ground--which means the the high frequencies are now escaping away from the signal path.

That's the way a passive guitar tone control works--the cap cuts the high frequencies by letting them pass through to the ground.

So in each case, small caps block more lower frequencies. It's just a matter of how they are used. Here's a simple calculator and illustration of high vs. low pass.

The other caps are cathode bypass and control grid caps for the tubes, and they're a little more tricky to explain...
DANIELEPO gmoon6 years ago
Thanks for you very good explanation, now the hole thing is more clear for me! Now comes the hard question. I have an amp that's almost the same as yours, it's a 803c, and because I'm a harp player I'd like it to be more "warm" (less bright). I found a lot of mods to warm the sound but no one tells you why the amp warms up. So, here is the question: what could be the frequency response of a "warm" amp? I know that the asymmetric distortion of the tube is also important obtaining "that sound", thats why I bought that type of amp, but I'd like to understand better the other variables. thanks for all
gmoon (author)  DANIELEPO6 years ago
It really difficult to quantify something like "warmth," since everyone has a different idea of what that means.

One thing tubes do is color sound with even-order (mostly second-order) distortion. It's a type of distortion closest to the fundamental sound itself, and is generally found to be pleasing to the ear. Mosfets can do this too, but somewhat differently.

But a good guitar tube amps also have odd-order distortion (third, fifth, etc.), which is the "bite" or "crunch" we love.

Frequency response itself is interesting, since a guitar (or harp) amp doesn't reproduce sound faithfully--as a stereo system they sound like crap. Plus a guitar speaker normally doesn't have a frequency response above 5kHz or so. But you will hear higher frequencies--they come from the speaker cone and the cabinet / baffle board vibrating at higher harmonics.

Just to clarify, when I wrote: small caps block more lower frequencies, that's a little misleading.

Caps block frequencies below the cutoff freq. Smaller caps don't block more lower frequencies--the cutoff frequency moves higher, so more mid-range cut is added to the already blocked frequencies below.

In the chart here of a high-pass filter, the cutoff would move to the right, as the cap value gets smaller.
It will become clear if we look at the formula for the REACTIVE resistance of a capacitor the formula is Xc=1/2*3.142*frequency* copacity IN FARAD Or as we say it one over two pie eff see Here you can see that the higher the frequency the lower the effective resistance and the larger the capacitor the lower the effective resistance
Ceiling cat5 years ago
Hi.  I was wondering if you could help me.  I have an old Lindell three tube amp and it's not working properly.  When I turn it on, as soon as the tubes warm up there is a VERY LOUD hum , and it's unaffected by the volume dial.  Could this be a bad filtering cap?
Yes bad filtering caps are usually the cause.
gmoon (author)  Ceiling cat5 years ago
Hi. Older amps almost always need new filter caps at some point. Loud hum regardless of volume does seem to point in that direction.

It could be other things, too, though. Sometime the tubes themselves develop hum and have to be replaced.

You could try pulling the preamp tube and then turning on the amp--unless it's a "radio tube" type amp that has all the filaments wired in series (like the amp in this instructable.) In that case the amp won't work with one tube pulled.

Always nice to have a few backup tubes anyway, so new tubes are good investment...
Thanks. I'm actually on my way out to Radio Shack for new caps.  As for the tube  configuration, I think it is how you said.  It's a very simple circuit.  And don't worry, I have plenty of tubes around.  I bought a bin of four tube radios and a box full of tubes at a swappers meet for $10.  Thats where I got the 12AU7 for my tube boost pedal.  I have like seven.
gmoon (author)  Ceiling cat5 years ago
Ok, cool, you've eliminated tubes as a problem.

Good luck finding the caps at RadioShack, though. The store near me doesn't carry electrolytics at the capacitance & voltage needed for a tube amp...(even for the relatively low voltage of "radio tube" amps-- 120V-150V.)
Unfortunately I found out it's the same story here.  I biked a few miles to RadioShack only to find they didn't have the caps I needed.  Not even the right capacitance.  It wasn't a total loss, though.  I got some electrical tape and solder that I needed badly.  And I got a ride home with a friend I ran into.
Oh, and earlier today I found a fuse holder with a mount for free.  I'll have to do the fuse/three-prong-plug mod sometime.
Good news!   Last week I bought some new caps at a local amp repair shop called "Mike Tech".  I soldered them in and took it back there and he formed them with a variac for free.  I brought it home, hooked it up, and the results were great.  It sounds beautiful!  It's got a great tube sound when I crank up the volume.  Very happy with it, and glad my dad's old amp is still in the family and finally working and being played again.
hg3414 years ago
my grandpa was move out to a smaller house and geting rip of most of his stuff a few years back and i took some thing that look like a ham raido

i hadnt realy look at it since then in till last weekend and found out in was a 300w tube amp by eico

im trying to get speakers for it now
gmoon (author)  hg3414 years ago
Pretty cool. Should be worth fixing up.

(don't run it without speakers...)
hg341 gmoon4 years ago
im sry i get all execited and grously over-estamate what some things can do....
its only 14 watts

buts its the famed hf-81

every review i saw was like this

gmoon (author)  hg3414 years ago
Definitely worth fixing up.

12-14 watts of tube power is LOUD. Loudness is logarithmic--subjectively, twice as loud as 14 watts is 140 watts.
hg341 gmoon4 years ago
on the back of it it has screws for the speakers marked g 4 8 16 and 32
what dose this mean(i think it the ohms but i dont know...) i do know what g is...
1st is what im talking about 2nd is a bouns
cgillyard hg3414 years ago
That is your speaker impedance terminal strip. Use them to connect your speakers to the corresponding impedance screws. For example if you have a set of 8 OHM speakers. Connect the positive (+) terminal of your speaker to the 8 OHM screw and the negative (-) terminal of your speaker to the 0 screw to the amp. Sometimes that 0 terminal screw on your amp is also known as C or common. Hope this helps. Let me know if anything. Thanks.
gmoon (author)  hg3414 years ago
Yeah, that's ohms. For an 8 ohm speaker, connect one wire to "8" and the other wire to "g" (ground).

Must be stereo, so two speakers (at least) are required--one per channel.

1st is what im talking about 2nd is a bouns

Sorry, I didn't get that...
hg341 gmoon4 years ago
ok im looking to buy some speakers for this what would be best 2 8 16 or 32?
gmoon (author)  hg3414 years ago
None is inherently better; all the better speakers come in different ohms loads. Just work with what's available.

You can also wire speakers together. Two (identical) speakers in series doubles the resistance; Two in parallel halves the resistance...
hg341 gmoon4 years ago
cool thanks for that

"1st is what im talking about 2nd is a bouns" i was talking about the pics
the 2nd one is the uncoverd bottom of my amp no real need to show but its still fun to look at.
i was wondering, what kind of output you could get from just one tube? I've been thinking about making a tube headphone amp but haven't seen any schematics anywhere (maybe i haven't looked hard enough), and i wasn't sure whether it would be plausible...maybe just a tube preamp and if the signal is still too weak a little bit of solid state amplification (i heard FET's sound more "tubie" then most opamps)...i should probably just look at some schematics and see how they manage the headphone jack
gmoon (author)  carpe_noctem4 years ago
A single tube is a little problematic, especially if you're looking for a classic tube sound.. A hybrid approach has been done--tube preamp with an FET or a LM386 to drive the headphones would work.

The other possible approach would be to use triode/pentode paired in a single tube. There are quite a few out there. You might even try one of the low-voltage tubes intended for cars and portable radios. This page has some possible combo tubes... These are safer to use than normal high-voltage tubes.
s3mamill4 years ago
I just got this exact same amp. I thought it would be a good starter project since its a simple circuit, but it seems over my head, since I have very little experience with electronics, wiring diagrams, etc. Now I'm considering some options: 1. Have it serviced by a pro. It would be safer and the work would be done right. Low risk, but higher cost. 2. tearing out what's there and installing a kit in the body. Higher cost, but doing the work myself. 3. Trying your mods and risk messing it up irrevocably. Low cost, but high risk.

When you said "I'd tear the amp down to the bare chassis and rebuild from there." did you mean rebuild from a kit or design a circuit yourself? Thanks for a great instructable!
gmoon (author)  s3mamill4 years ago
Hey, that looks good.

By "tear it down and rebuild," I meant that a less "hacked" approach would probably lead to a cleaner, more organized, less cluttered circuit. As-is, it's gonna be more difficult to replace the caps in 30 years. The tangled wiring is more fragile than a well-planned strip-down / rebuild, too.

However, it's extremely quiet and sounds great, so I'm not too disappointed.

I'm not sure what approach is right for you. If you try rebuilding it yourself (and fail), you can always take it in to a profession to get it sorted. Just be REALLY CAREFUL if you do.

If you're not confident you can do this safely, take it to an amp tech...
Man, it's crazy that you posted this because I was just wondering why my old Gibson Falcon amp shocked me when I was barefoot! How long do the filter caps hold a charge?
gmoon (author)  FireandIcebike7 years ago
Caps can hold a charge for a loooooong time (although the older ones usually don't, 'cause they're failing.) Were you pokin' around inside or just playing? Isolation can be a problem with these oldies, too.

I've been dumb enough to play my old Ampeg in the basement with barefeet....wasn't a painful shock, I could just feel the current flowing from my left hand to the floor. One lesson learned.
I was just playing. Man, I didn't think an amp could be all lethal and junk! Thanks for the info.
gmoon (author)  FireandIcebike7 years ago
Yeah, actually quite a few people have died from this stuff. Very likely your guitar is wired to use you as a ground (through the strings.) On some old amps, the chassis ground might instead be a lethal voltage in respect to another 'ground' (you in your bare feet, or a mic stand, for instance.) There's a little info on Step 3 about that, if it's an amp without an isolation transformer. But even the amps with isolation can be dangerous--that's why I added the three-prong cord here....
gmoon (author)  gmoon7 years ago
Here's an illustration of an older amp with an isolation transformer that still dangerous-- This is part of a schematic of my Ampeg Gemini II.

Because AC line plugs were all two prong, the amp has a two-position standby switch. This switch not only works as a standby, but it also connects the AC line to the chassis through a single capacitor.

Since the plugs weren't polarized, you could use this to connect the chassis ground indirectly to the line 'neutral' wire, and your amp would run quieter. But if the cap degrades and fails, and your guitar strings are grounded (as most are)--it's like poking your finger in the wall socket...

On cheaper amps without a standby switch, the cap was still there, but you'd change the plug direction to ditch the hum...

(picture: red square shows the switch and the 'death cap.')
 Shocking to say the least, especially when I'd be playing my guitar then would touch my friends at the same time...YIKES!!!!  that GeminiII....
gmoon (author)  satanalingus5 years ago
That Gemini cured me of ever playing guitar in bare feet...
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